A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus, a measurement system is used for determining the position of a substrate stage with high accuracy (e.g. nanometer accuracy). Due to a continuing demand for higher throughput and increased accuracy, it is desirable to improve the accuracy of measurement systems used in the lithographic apparatus, in particular for the measurement systems with which the position of the substrate stage and reticle stage are measured, and typically in six degrees of freedom.
In an embodiment of the measurement system, an encoder type measurement system is used. Such encoder-type measurement system may comprise one or more sensors mounted on the movable object and at least one sensor target object, for instance a sensor target plate comprising a grating or grid, the sensor target object mounted on a substantially stationary frame, in particular a so-called metrology frame (metro-frame). The sensor target object may comprise a one-dimensional or multi dimensional grating. In an embodiment, the sensor target object will be typically in the form of a plate on which a two dimensional orthogonal grid is arranged. Such sensor target object is often referred to as grid, grating or grid plate.
In alternative embodiments, the one or more sensors may be mounted on the substantially stationary frame and the sensor target object may be mounted on the movable object. The grid sensor target object, for instance grating or grid plate comprises grid lines or other grid markings which are used to determine a change in position of the grid plate with respect to the one or more sensors.
Position control is carried out by measuring the position of the sensor with respect to the sensor target object in one or more degrees of freedom. The grid plate may comprise irregularities such as manufacture errors, local pollution or damage which may cause errors in the position measurement. Also, droplets of the liquid of an immersion type lithographic apparatus on the grid or grating may cause errors in the grid or grating. In the context of the present application local pollution, damage and droplets or particles which cause an error in the grid or grating are regarded to be errors.
The presence of an error on a grid or grating may lead to incorrect positioning of the patterning device support or substrate table with respect to the optical axis of the lithographic apparatus. As a result, the image of a patterned beam of radiation which is projected on a target portion of the substrate is not correctly positioned on the substrate. Such imaging errors such as overlay and/or focus errors, may lead to inferior product quality, or even complete rejection of the substrates. In this respect it is remarked that the quality of the wafers may only be determined after complete production of the wafers. Therefore, the presence of grid or grating errors may result in a large impact on the production of the lithographic apparatus. Thus it is desirable to take grid or grating errors into account.
In lithographic apparatus, it is known to calibrate the grid or grating. In such calibration method, the complete grid or grating is scanned on irregularities. A map of the grid or grating is made of any irregularities such as errors. This calibration map is used during actual position measurement to correct incorrect measurements due to the presence of errors.
The calibration of the grid or grating may improve the position measurement of the position measurement system since the known errors are taken into account. However, errors which come into existence after the calibration step, such as pollution, damage, droplets or particles, will not be known in the correction map. The presence of such errors may still lead to the imaging errors in a large number of substrates. To avoid the risks on bad product quality of the substrates, the calibration step may be repeated regularly to map new errors in the calibration map. However, increasing the number of calibrations may have a negative influence on the overall throughput of the lithographic apparatus.